The purpose of this study is to elucidate key steps in the processes leading to dental caries. Although major features of this disease, i.e. the involvement of specific acidogenic microorganisms, acid production and a concomitant decrease in pH followed by enamel demineralization are well established, several critically important aspects remain unclear. Of particular importance are the degree of saturation with respect to enamel mineral of the extracellular aqueous phase of dental plaque, plaque fluid, and the factors which influence this property. These factors include the types and amounts of organic acids produced, increases in calcium and phosphate concentrations via the dissolution of plaque solids, the inherent buffer capacity of plaque, and the production of ammonia. The proposed study will provide direct measurement of the physical chemical factors involved in caries initiation and provide a much-needed link between these factors, microbiological determinants and caries susceptibility. To resolve these problems we propose to determine the total concentration of individual organic acids (acetic, lactic, formic, butyric, propionic, succinic) and inorganic ions (Na+, NH4+, K+ Mg2+, Ca2+, Cl-, phosphate, and carbonate) in pooled plaque fluid, before and at selected times following exposure to sucrose; the fluid will be obtained from plaque associated with sound, white-spot, and carious enamel from individuals with well-defined caries histories. Using the analytical data obtained, degrees of saturation with respect to enamel mineral and calcium phosphate phases will be calculated. The microbiological composition of the collected plaque by conducting in vitro enamel demineralization experiments using solutions mimicking the plaque fluid compositions found for cariogenic and non- cariogenic plaques. Furthermore, we propose to determine the effect of stimulated salivary flow on the composition and properties of plaque fluid. In addition, we will also investigate the inherent buffer capacity of plaque fluid and whole plaque, and explore the possibility of calcium phosphate mineral phases in plaque as potential buffers and sources of calcium, phosphates and fluoride ions, following acid production. This approach will provide a unified picture of the caries process and may ultimately be useful in the design of anti-caries procedures and in the assessment of patient risk.